When I used to do small/medium client work, I used Kirby CMS (For context, Kirby is a flat file CMS). None of them needed the power or flexibility of MySQL or even SQLite, so it was perfect. PHP scripts can still filter the flat files to create collections of categories or tags, so they didn't miss anything since it's not a static site. No need to recompile if you wanted to add a tag to everything. PHP is even faster today but back then it was still fast enough for basic pages and blogging.

I highly recommend it even though it's not free because it's highly refined and actively developed year round because of their pricing model.